The central objective of this thesis is to better understand early life history dynamics of salmonids in large regulated rivers. I studied spawning, incubating, and age-0 life stages of rainbow trout in the Lee’s Ferry reach of the Colorado River below Glen Canyon Dam, AZ. My first objective was to evaluate the effects of hourly fluctuations in flow on nearshore habitat use and growth of age-0 trout. Catch rates in nearshore areas were at least 2- to 4-fold higher at the daily minimum flow compared to the daily maximum and indicated that most age-0 trout do not maintain their position within immediate shoreline areas during the day when flows are high. Otolith growth increased by 25% on Sundays in one year of study, because it was the only day of the week when flows did not fluctuate. My second objective was to evaluate the effects of flow fluctuations on survival from fertilization to a few months from emergence (early survival). Fluctuations were predicted to result in incubation mortality rates of 24% in 2003 and 50% in 2004, when flow was experimentally manipulated to reduce trout abundance, compared to 5% in 2006 and 11% in 2007 under normal operations. Early survival increased by over 6-fold in 2006 when egg deposition decreased by at least 10-fold. Because of this strong compensatory dynamic, flow-dependent incubation mortality in experimental years was likely not large enough to reduce the abundance of age-0 trout. My final objective was to determine how flow, fish size and density effects habitat use, growth, and survival of age-0 trout. Apparent survival rates from July to November were 0.18 (2004), 0.19 (2006), and 0.32 (2007). A stock synthesis model was developed to jointly estimate parameters describing early life history dynamics, and indicated that early survival was lower for cohorts fertilized during the first half of the spawning period and was negatively correlated with egg deposition, that movement of age-0 trout from low- to high-angle shorelines increased with fish size, and that survival varied by habitat type and over time in response to flow changes from Glen Canyon Dam. / Science, Faculty of / Zoology, Department of / Graduate

Many factors affect membrane composition in ectotherms, including allometry, temperature, toxins such as PCB-153, and osmotic stress. This thesis seeks to describe the relationship between membrane composition, size, and phylogeny in twelve species of cypriniform fish; to describe interactions between the homeoviscous responses to temperature and to PCB-153 in goldfish and rainbow trout; and to describe the membrane response to hypoosmotic stress in goldfish. Commonalities in these patterns provide insight into shared mechanisms of phospholipid modulation. In particular, such similarities indicate whether the membrane pacemaker theory of metabolism, which connects allometric relationships between body size, membrane phospholipids, and metabolic rate, can serve as a general framework for understanding membrane composition. Chapter 2 investigates how cypriniform membrane unsaturation decreases with mass through different fatty acid substitutions than in endotherms, but these fatty acids are in turn shown to be due to the species’ relatedness to one another rather than to purely physiological causes. In Chapter 3, PCB-153 is shown to increase cholesterol in liver and brain, while high temperature primarily reduces phospholipid unsaturation. In Chapter 4, these patterns are further explored in trout. As in goldfish, cholesterol modulation is the primary response to PCB-153, whereas temperature primarily reduces phospholipid unsaturation. Trout show more pervasive fatty acid changes than goldfish in all tissues except the liver, which does not respond to PCB exposure, suggesting that PCB-153 pushes trout’s homeoviscous response to a limit that similarly-exposed goldfish do not face. Chapter 5 shows that goldfish intestines decrease membrane saturation; kidneys decrease membrane cholesterol; gills decrease neither; and muscles decrease both in response to long-term exposure to hypoosmotic conditions. The intestine and kidney are both involved in recovering ions from body fluids, but gills suppress ion loss and muscle concentrates ions from the bloodstream. Temperature, osmotic stress, PCB-153, and increasing body size are all addressed via a similar set of membrane responses in fish, which fits with the membrane pacemaker theory’s predictions regarding membrane composition, metabolic rate, and size.

There is an increasing scientific acceptance that fish may feel some sort of fear, pain and distress, which in turn feeds a growing concern for their welfare. Humans impact the wellbeing of a large number of fish in various ways, one of them being through research. Welfare legislation in the UK demand welfare considerations for all animals used in scientific procedures. Furthermore, welfare and enrichment needs for fish are included in the Appendix A of the European Convention for the Protection of Vertebrate Animals used for Experimental and Scientific Purposes. As fish are extensively used in research, changing their housing and husbandry to improve welfare is of importance, since fish kept in laboratories are most likely subjected to impoverished environments. Although enrichment programs have been shown to improve health and welfare in various animal species, little is known of their potential for application to juvenile rainbow trout. How best to improve barren experimental tanks for female juvenile rainbow trout used in regulatory research was the broad aim of this PhD. In this thesis, three enrichment strategies for rainbow trout have been examined, using physiological and behavioural welfare indicators. The first study assessed the effects of semitransparent shelters on trout welfare, and a clear message became evident; that shelters of this design should not be considered enrichment for rainbow trout as they had several significant negative impacts, indicating chronic stress in fish from shelter tanks relative to fish in a barren environment. The second study investigated impacts of reduced visual access to conspecifics in the same tank. Habitats with low visual contact between individuals have been suggested to reduce aggression for a range of species, and I have shown that visual barriers appeared to be beneficial to trout as well. The final experiment evaluated effects of high and low water currents on the wellbeing of rainbow trout, and results indicated increased fish welfare when water currents were supplied.

Competition is defined as the demand of two or more organisms for the same extrinsic resources in excess of supply. The distribution, movements, behaviour and food of trout and shiners in Paul and Pinantan lakes were studied in order to determine the item's and mechanisms of interspecific competition between them. Data from recent years were compared with data for years when trout alone inhabited the lake. No interspecific aggression was observed. The possibility that the two species were competing for space was discounted. Stomach contents of shiners in Pinantan Lake revealed a marked qualitative diurnal food cycle. In Paul Lake, shiners have drastically reduced the Gammarus population relative to its pre-shiner abundance. This overgrazing was caused by the concentration of large numbers of shiners over the shoals where Gammarus are also present in their highest concentrations and the ability of shiners to pursue food deeper into the weeds and to graze an area more thoroughly than trout. In Pinantan Lake shiners have apparently-reduced the density of Daphnia to a point where trout are unable to feed on them as rapidly as in pre-shiner years. The ability of both species to utilize many types of food tends to reduce the intensity of competition. The study demonstrates how false implications may arise from a delayed appraisal of competition. If observations had not been made on Paul Lake until after competition had been observed the importance of Gammarus as an item of competition would have probably been overlooked and the whole competitive relationship misconstrued. Included among the basic mechanisms of competition is the consumption by one or more organisms of something in short supply before it reaches a potential habitat where it would become available to another organism or group. Environmental factors and behaviour were shown to be important influences in the dynamics of competition. The physical and biological environment and the distribution and behaviour of competitors may be in states of continual flux. Hence natural competitive relationships can be considerably more complicated and variable than situations described by the most elastic of theoretical models. / Science, Faculty of / Zoology, Department of / Graduate

Production of the glucocorticoid hormone cortisol in response to a stressor is initiated by activation of the hypothalamic-pituitary-interrenal (HPI) axis in fish. Serotonin (5-HT) and negative feedback regulate cortisol production at the whole-animal level; the objective of the present thesis was to investigate their roles in regulating cortisol production by interrenal cells of rainbow trout (Oncorhynchus mykiss). Messenger ribonucleic acid (mRNA) for the 5-HT4 receptor was present in low abundance in interrenal cells. In addition, cortisol production was significantly increased for in vitro head kidney preparations incubated with 5-HT, and this elevated cortisol production was blocked by the 5-HT4 receptor antagonist 5-fluoro-2-methoxy-[1-[2-[(methylsulphonyl) amino] ethyl]-4-piperidinyl]-1h-indole-3-methylcarboxylate sulphamate (GR125487). Thus, 5-HT acts at the head kidney level to regulate cortisol production, probably via the 5-HT4 receptor. Chronic social stress did not appear to regulate the expression of key proteins involved in cortisol biosynthesis or corticosteroid receptors (CR). However, head kidney tissue incubated in vitro with cortisol for 2-8 h showed a reduction in adrenocorticotropic hormone-stimulated cortisol production compared to controls, suggesting the existence of an ultra-short-loop negative feedback mechanism. Thus, the high circulating levels of cortisol in trout experiencing chronic social stress may activate this ultra-short-loop negative feedback mechanism to suppress cortisol production at the head kidney level.

Stunting is widespread among brook trout Salvelinus fontinalis populations in high alpine lakes in the eastern Sierra Nevada, California. Due to their small size and poor condition, stunted brook trout are undesirable as sport fish. In the same area, a few lakes contain large brook trout. Population density was the primary difference between lakes with different sized fish. I hypothesized that in lakes with large fish the food ration per individual was sufficient and that in lakes with stunted fish the food ration was the limiting factor. I carried out removal experiments on eight brook trout populations to test the hypothesis (1) that fish size is inversely related to population density, and by that evaluate density reduction as a means of improving growth in stunted brook trout. I considered seven additional hypotheses regarding the relationships between brook trout population density and growth, fecundity, and recruitment: (2) growth response is proportional to density reduction; (3) growth response is inversely proportional to pre-reduction density; (4) growth responses of juvenile and senescent fish are less affected by density reductions than mature, reproductively active fish; (5) growth response to density reduction is inversely proportional to lake elevation; (6) fish size is proportional to angling pressure; (7) fecundity response is proportional to the reduction in population density; and (8) recruitment response is inversely related to density. I used gillnets to simultaneously remove part of the population and estimate population size through catch depletion methods that allow variable catchability. Catchability varied with lake size and with abundance, increasing as population abundance declined. Increased catchability can be explained by behavioral responses. I measured and aged 16000+ brook trout from 71 lakes, 9800+ from the eight experimental lakes. I validated annual structures on otoliths using a fluorochrome mark. For the experimental lakes, I back-calculated previous population sizes using estimates of number at age in 1989, catch at age in 1987-1988, and survival rates at age estimated from catch data collected in 1987-1989. I converted population estimates into density estimates of fish and biomass per lake surface area and volume. I tested hypothesis 1 by using survey data from 61 populations and by experimentally manipulating density in eight populations. The survey data suggested that size differences between populations of brook trout are a function of population density. Results from the eight removal experiments showed that fish size was inversely related to population density, though the increases in fish size were minor. The relationship between change in length and weight was roughly proportional to the change in density (hypothesis 2). Hypothesis 3 suggested differences in the severity of stunting in alpine lakes, and that the growth response of severely stunted populations would be more pronounced than the response of less stunted fish in lower density populations. The result was opposite; the growth response in lower density populations was greater than the response in higher density populations, suggesting that the growth response may have been proportional to the pre-reduction density. Hypothesis 4 suggested that the growth response for juvenile brook trout would be less than that for the pre-senescent adult population. The results refuted the juvenile portion of hypothesis 4: response for juveniles was greater than the response of the adults, perhaps because of greater recuperative abilities in young fish. The data supported the hypothesis that the growth response would be diminished in older fish. There was no relationship between elevation and growth response (hypothesis 5). Sport fishing had little effect on the growth of brook trout populations (hypothesis 6). Heavily fished populations were also stunted. Stunted brook trout had fecundities similar to non-stunted brook trout of the same size (hypothesis 7). Individual fecundity did increase in response to density reduction, but no more than would be expected from the increase in size. In several populations mean absolute fecundity decreased with age. Ovary weight was maintained by an apparent increase in mean egg size in older fish. The recruitment response varied between lakes (hypothesis 8). Recruitment did increase, likely in response to reduced cannibalism or competition, but I also found recruitment failure at the highest levels of density reduction. Strong cohorts were produced by increased juvenile survival rather than increased population fecundity, since population fecundity had decreased due to removal of most of the adult population. In one lake with almost no recruitment, densities remained low and fish weight doubled. For density reduction to be an effective means of increasing fish size, recruitment must be inhibited. / Science, Faculty of / Zoology, Department of / Graduate

Les cellules souches spermatogoniales (SSCs) constituent la population de cellules germinales initiales support de la production des spermatozoïdes tout le long de la vie d’un individu. Ces cellules caractérisées par leur capacité d’auto-renouvellement et de différenciation maintiennent ainsi une réserve et garantissent la production continue de cellules germinales différenciées. Chez les mammifères, plusieurs marqueurs permettant de reconnaitre cette population cellulaire au sein du testicule ont été identifiés. De plus, parmi ces marqueurs, certains permettent d’isoler et de purifier les SSCs. Ils ont ainsi permis d’aborder les mécanismes de régulation du devenir des SSCs par des expériences menées in vitro et in vivo. En revanche, la biologie de ces cellules est beaucoup moins connue chez les vertébrés non mammaliens, en particulier chez les poissons téléostéens. Notre modèle d’étude, la truite arc-en-ciel (Oncorynchus mykiss) est caractérisée par une spermatogenèse cyclique et fortement saisonnée. La croissance du testicule immature, la prolifération active des spermatogonies à la puberté, et la quiescence de ces dernières en fin de cycle semblent être des étapes clés de régulation du devenir des SSCs. Grâce à l’analyse des profils d’expression au cours du cycle spermatogénétique et dans des fractions de cellules testiculaires isolées, nous montrons la conservation d’expression de gènes décrit comme marqueurs de SSCs chez les mammifères (pou2, plzf, nanos2 et 3, gfra1) dans les populations de spermatogonies A indifférenciées de truite. En particulier, gfra1 et nanos2 identifient tous deux une sous-population de cellules au sein des spermatogonies. Nous proposons donc que les cellules gfra1+ et/ou nanos2+ sont des SSCs au sein du testicule de truite. Par ailleurs, nous montrons que l’orthologue truite de gdnf, ligand de gfra1 et régulateur majeur du maintien des SSCs chez la souris, est exprimé très fortement juste avant la fin de cycle spermatogénétique. Cette expression corrélée avec un pic de sécrétion plasmatique de Fsh suggérait une régulation positive de gdnf par cette hormone. Notre étude in vitro n’a pas permis d’aboutir aux mêmes conclusions, mais cette technique ne reflète pas toutes les régulations réciproques ni le rôle des autres facteurs in vivo. En conclusion, nous avons découvert des marqueurs probables de SSCs chez la truite. En particulier, gfra1 et nanos2 qui permettront une analyse plus approfondie de la biologie des SSCs chez les téléostéens. De plus, l’expression de gdnf et de son récepteur dans le testicule, régulée en fonction du stade du cycle spermatogénétique, nous permet d’envisager cette voie comme régulateur du devenir des SSCs chez la truite. / Spermatozoa production throughout life requires the presence of the initial germ cells, the spermatogonial stem cells (SSCs). Their self-renewal and their ability to differentiate assure to keep a reserve and to produce continuously differentiated germ cell. In mammals, several markers of the SSCs have been identified. Interestingly, some of them allow us to sort the SSCs population and further to analyze their fate in vitro and in vivo. By contrast, only scarce information has been obtained in non-mammalian vertebrates including the teleost fishes. Our model of study, the rainbow trout (Oncorynchus mykiss), presents a seasonal spermatogenesis. The growth of the immature testes, the active spermatogonial proliferation starting at puberty, and their quiescent state at the end of the cycle represent interesting stages to study the regulation on the SSCs fate. Using various testicular stages and purified testicular cell fractions we show that pou2, plzf, nanos2 and 3 and gfra1, all expressed by spermatogonial stem cells in mammals, are specifically expressed in the undifferentiated A spermatogonia population. In particular, gfra1 and nanos2 are expressed in a sub-population of these cells. Thus, we propose that the nanos2+ and/or gfra1+ cells are SSCs. Furthermore, in our study, gdnf, ligand of gfra1 regulating the SSCs fate in mouse, is highly expressed just before the end of the spermatogenetic cycle. Such expression correlates with Fsh peak of secretion. However a stimulation of gdnf expression by Fsh was not observed in our in vitro experiments, but this technique doesn’t represent reciprocal regulations nor the roles of all factors in vivo. To conclude, we discovered potent marker of SSCs in trout. In particular, gfra1 and nanos2 will allow us to investigate further the SSCs biology in teleosts. Moreover, gdnf and its receptor expression in the testis in a spermatogenetic-dependent way lead us to propose this pathway as a potent regulator of the SSCs fate in trout.

The inter-tissue metabolic responses to hypoxia were determined in lungfish (Protopterus aethiopicus), and trout (Salmo gairdneri). Lungfish respond to hypoxia with a reduction in metabolic rate. It was intended to determine which tissue, or tissues exhibit decreased metabolic rates during hypoxia, and then compare the results with the metabolic reactions observed in trout, which are not reported to reduce metabolic rate during hypoxia. The metabolic potentials of the heart, brain, white muscle and liver in the African lungfish were estimated using enzymatic data. Metabolic effects of a 12 hr submergence were monitored using metabolite measurements. Heart was the most oxidative tissue, but also showed the greatest anaerobic potential. The brain displayed relatively low oxidative capabilities. White muscle remained almost inert. Although high energy phosphate concentrations in brain and heart did not fall during submergence, glycolysis was activated as indicated by cross-over plots, depletion of endogenous glycogen stores, and lactate accumulation. Blood-tissue lactate and glucose gradients indicated (1) that the heart and brain released lactate throughout submergence, (2) that after 12 hr of submersion the brain and heart were probably obtaining all their required glucose from the blood (3) that the liver released glucose throughout submergence, and (4) the white muscle was metabolically isolated from the rest of the body during submergence. The lack of measurable changes in white muscle metabolite concentrations coupled with the low enzyme activities leads to the suggestion that the most significant adaptation to hypoxia in these fishes may not be the capacity for increased anaerobic energy production. Instead, it is likely that the ability of the muscle to prevent the activation of glycolysis during hypoxic dysoxia is the key to the animal's survival. Histochemical and ultrastructural studies were done on the axial musculature of the lungfish. The small wedge of red coloured muscle evident upon gross examination was shown by histochemical demonstrations of lactate and succinate dehydrogenases, of adenosine triphosphatases, and of lipid to be composed of a mosaic of red and intermediate fibres. Respectively, these fibres measured 23.6 and 34.3 microns in average diameter. The bulk of the myotome is composed of white fibres having an average diameter of 67.3 microns. Mitochondrial density, capillarity and lipid content were very low for all fibres. These data suggest that the axial musculature is geared primarily for anaerobic function. The relatively large percentage of white muscle indicates that the overall metabolic rate of the axial muscle is low. The capacity of the muscle to exist with a reduced rate of ATP turnover (as was suggested above) may be related to the large proportion of white fibres present in the myotome. Tissue metabolites were measured in a hypoxia sensitive organism, the Rainbow trout (Salmo qairdneri), before and after exposure for 3 hr to inspired oxygen tensions of 20 torr (at 4°C). There were small changes in the brain but the energy status was maintained. The red muscle was the least affected. White muscle creatine phosphate was depleted. Various data indicate that the white muscle is the major user of glycolytic substrates and the major producer of lactate. The heart is stressed as indicated by a decline in glycogen, ATP, CrP, and the total adenylate pool. The liver exhibited declines in every indicator of metabolic homeostasis. The liver concentrations of glycogen did not decline. The fact that anaerobic glycolysis has been activated in the white muscle, while the muscle remains in metabolic communication with the other tissues via the blood, supports the suggestion that the metabolism of the white muscle will have a pronounced effect on the metabolic status of the whole animal. The trout is maintaining its rate of oxygen uptake while activating anaerobic glycolysis in the attempt to maintain 'normal' rates of energy utilization. The turnover rates of glucose and lactate were measured in trout subjected to the same hypoxic stress as above. Glucose turnover did not change while lactate turnover increased from 2.8 ± 0.4 µmoles/min./kg to 20.6 ± 6.8 µmoles/min./kg. The lack of increase in glucose turnover was attributed to the observation that liver glycogen concentrations do not change and so there is no increase in glucose flux. The increase in lactate turnover emphasizes the fact that anaerobic glycolysis is activated and that some tissues are oxidizing lactate. The problem of when a cell becomes hypoxic and the reactions of the cell to that stress is addressed. The cell (tissue, organ, animal) has two options if oxygen supply drops to a level which prevents oxidative metabolism from supplying all of the requirements for ATP synthesis. The cell may exhibit a decline in requirements, in which case the rate of ATP production need not be as high as in the oxidative state or, conversely, anaerobic energy production may increase in the attempt to maintain ATP production rates. The lungfish muscle appears to be capable of the former, thus preserving substrates for other tissues and reducing the rate of end-product formation. The trout white muscle, on the other hand, exerts a major influence upon the other tissues when the animal is stressed with hypoxia. The term 'energy conformer' is applied to animals which do not maintain oxygen uptake in the face of a declining supply, and which allow ATP production to decline concomittantly by not activating glycolysis to a marked degree. An energy regulator would activate glycolysis in the attempt to maintain oxidative rates of ATP production. The trout is more of an energy regulator than is the lungfish with the main difference in this capacity being in the white muscle. / Science, Faculty of / Zoology, Department of / Graduate

An analysis of the systematics, zoogeography and evolution of the Dolly Varden char species complex in British Columbia is presented. These features of this species complex and the morphometric statistical procedures used in these analyses have both long been the subjects of strong debate and also have recently seen much renewed interest and work. This thesis assesses both these areas and is divided into those two parts. The first section deals with these three biological topics, and the second section contains a synthesis and exploratory data assessment of the commonly used morphometric techniques and provides some new methodology for understanding their requirements and interpreting their results. PART I 1. The systematics of the Dolly Varden char species complex is examined by using principal component analysis (PCA) to designate typological species groupings and then employing linear discriminant function analysis on a reduced set of significant characters to classify the remaining specimens. This typological distinction is verified with distributional information that reveals no interbreeding of the species in areas of parapatry and sympatry, and with preliminary information regarding intra- and inter- specific crosses, spawning colouration, skull osteology, cytology and embryology. This data is also suggestive of competitive exclusion and character displacement. All these results indicate that the Dolly Varden char species complex in B.C. is composed of two species, Dolly Varden (Salvelinus malma) and bull trout (Salvelinus confluentus). 2. The zoogeography of these two species is analyzed using canonical trend surface analysis (CTS). CTS can potentially separate confounding non-geographic morphometric information from the data and thus could allow historical zoogeograpbic patterns to be inferred from that data which corresponds to geography. Such a reconstruction reveals the possible glacial refuge origins and post-glacial recolonization patterns of these two species for each of the major river drainages in B.C.. 3. The evolution of these two species is assessed through the implementation of PCA to fit the cross-sectional morphometric data to an ontogenetic model. The resultant PCA size and shape vectors effectively portray allometric trends which indicate that Dolly Varden could have evolved from bull trout through neotenic paedomorphosis. This result is supported with data on growth rates and developmental homeostasis. PART II 4. A synthesis of the available but widely scattered and disparate information on the data and statistical requirements for morphometric statistics reveals the analytical problems that can result from not approximating underlying test assumptions. These assumptions are important, but are not appreciated or often assessed. Simple recommendations and rarely used tests for dealing with these requirements are provided. 5. The effectiveness and compatability of four bivariate morphometric techniques (ratios, log₁₀ ratios, allometric regression, regression residuals) are assessed. All methods provide similar but ineffective individual ordination and group separation. Their effects on characters differ greatly and are often unrealistic. None of these methods effectively removes all the confounding allometric size information, but allometric regression will usually be the best bivariate procedure. 6. A similar assessment of four multivariate morphometric procedures (covariance matrix PCA, correlation matrix PCA, shear matrix PCA, size-constrained matrix PCA) is undertaken. Size-constrained PCA results in non-orthogonal vectors that also do not represent the traditional multivariate morphometric size and shape vectors. As well, the character and individual information it provides is unrealistic. The other three techniques result in similar and effective individual ordination, group separation and removal of confounding allometric size information. PCA on a covariance matrix is likely the best multivariate method since it provides the most realistic size adjustment and character information. 7. PCA is often carried out on data which has been previously adjusted through bivariate procedures. An examination of this method demonstrates that it results in no benefits since the multivariate morphometric size and shape vectors are lost, and the data variation is no longer synthesized into only two or three resultant significant vectors. 8. PCA is also performed on mixed character data sets (continuous and discontinuous data). An assessment of this procedure shows that it provides improved group separation, but the representation of characters, individuals and multivariate morphometric size and shape relationships is confounded and unrealistic. There also is a slight reduction in data synthesis. 9. A methodology for back-transforming PCA output into the original and more intuitively comprehensible data scale, format and dimensions is given. This back-transformation also verifies the traditional belief that the first resultant PCA morphometric vector is size and that the second is shape. Separate unconfounded matrices for size and shape information in which only the significant data variation is accounted for can thus be independently back transformed. / Science, Faculty of / Zoology, Department of / Graduate

Rainbow trout (Salmo gairdneri) from two wild stocks native to British Columbia, and a non-native domestic strain were reared under varying conditions to examine effects of growth rate and body size, genetic stock, and photoperiod on the incidence and timing of testis development. The ratio of gonad weight to body weight (Gonadosomatic index) was found to be useful for separating mature and immature male fish and for determining the onset of gonadal development. In Premier lake fish testis development began one year before the expected date of spawning. At this time, signs of maturation were evident primarily among males that reached a body weight of between ten and twenty grams, whereas most of the fish smaller than this "critical" size remained immature (i.e. no testis developmment). Apparently, this critical size must be reached by a certain time of the year. Thus, a time 'window' exists wherein fish achieving a certain size begin preparation for spawning the following year. Altering photoperiod regime during the time window had no effect on the incidence of sexual precociousness, but did delay spermatogenesis by an undetermined length of time. / Science, Faculty of / Zoology, Department of / Graduate